QtIcoHandler::canRead(): avoid checking more than once

QtIcoHandler::canRead() calls ICOReader::canRead(), which assumes that
QIODevice::pos() is at the position where the .ico data starts (i.e.
pos() == 0 if this is a separate .ico file). But if an AnimatedImage in
Qt Quick opens an .ico file, canRead() gets called multiple times:
the first is when QQuickAnimatedImage::frameCount() eventually results
in QImageReaderPrivate::initHandler(); then ICOReader::readHeader()
is called, which moves the file position. The second time is when
QQuickAnimatedImage calls QMovie::isValid(). At that time,
QIODevice::pos() == 6: we need to avoid calling ICOReader::canRead()
because it's no longer at the start of the data.

Without this change, AnimatedImage reports "Error Reading Animated Image
File" and doesn't show anything.

The fix is to store the known-good state, the same way that
QTiffHandler::canRead() returns true if its d->tiff already exists
(TIFFClientOpen() succeeded).

The test checks that it's ok to call QMovie::frameCount() first and then
QMovie::isValid(). Calling frameCount() has the effect of moving
QIODevice::pos().

Pick-to: 6.5 6.6 6.7
Task-number: QTBUG-117429
Change-Id: Ie3a5225f2cea9a0d76d685e83ce4d4a10cbe9188
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/plugins/imageformats/ico/qicohandler.h b/src/plugins/imageformats/ico/qicohandler.h
index 0fe251ab60..61c3eea465 100644
--- a/src/plugins/imageformats/ico/qicohandler.h
+++ b/src/plugins/imageformats/ico/qicohandler.h
@@ -30,7 +30,7 @@ public:
 private:
     int m_currentIconIndex;
     ICOReader *m_pICOReader;
-
+    mutable bool knownCanRead = false;
 };
 
 QT_END_NAMESPACE